1. Field of the Invention
The present invention relates to an optical instrument having a plurality of focus lens groups, and a drive section capable of independent drive for each of the plurality of focus lens groups, and to a lens control method.
2. Description of the Related Art
An imaging device such as a digital still camera or digital video camera generally has an autofocus function. Using this autofocus function, positional control is carried out so that position of a focus lens within a shooting optical system becomes an in-focus position. Also, in order to suppress the occurrence of aberration at the time of focusing, a lens with which each of a plurality of focus lens groups move differently from each other at the time of focusing, a so-called floating system, is frequently used. Further, a lens barrel type provided with independent actuators for a plurality of focus lens groups is provided in Japanese Patent Laid-open No. Hei 4-14005 (hereafter referred to as prior art literature 1).
The focus system described in prior literature 1 comprises a magnifying zoom lens, and a plurality of focus lens groups made up of first and second focus lenses, and each focus lens is driven by an independent actuator. At the time of a shooting operation, this focus system carries out positional control of each focus lens group at fixed time intervals, and can focus on a desired subject. In this positional control, it is necessary to drive the first and second focus lenses by independently calculating target positions, and the amount of calculation for calculating target positions of each focus lens becomes significant. Also, with an increase in calculation amount, it is not possible to drive each focus lens group rapidly.
In the case of a variable focus lens such as that described in prior art literature 1, if focal length is changed by moving the zoom lens position, then positions at which the focus lens groups focus on the subject will change, even at the same subject instance. Further, in the case of having a floating focus configuration comprising a first focus lens and a second focus lens, cases may arise such that respective focal positions for a subject corresponding to focus length are not identical between the first focus lens and the second focus lens.
This means that when carrying out a magnification operation during shooting, there is a need for positional control to make in-focus position of the focus lens groups track in accordance with the magnification operation. This positional control is called a zoom tracking operation. To carry out zoom tracking, first, at fixed time intervals a focus lens group other than the first focus lens is instructed to move to a position that has been designated by the first focus lens and a position determined from focal length (first control). Then, if a magnification operation is performed, the position of the focus lens group other than the first focus lens becomes a different position to the position of the first focus lens after the magnification operation and the position determined from focal length.
If the above described magnification operation is carried out, a position where the focus lens group focuses on a subject is indeterminate. Accompanying this, a state arises where a taken image is out of focus in a period until position synchronism returns. Also, before and after a magnification operation a target position of a second focus lens is moved by rapid changes. This means that in a case where position of the second focus lens is subjected to tracking movement in conformity with movement of target position of the second focus lens accompanying a magnification operation, movement acceleration of the second focus lens changes rapidly. As a result, noise and vibration occur when the second focus lens is made to move, due to counteraction at the time of movement.